Dry etching method

ABSTRACT

A dry etching method according to the present invention includes etching silicon nitride by bringing a mixed gas containing hydrogen fluoride and a fluorine-containing carboxylic acid into contact with the silicon nitride in a plasma-less process at a temperature lower than 100° C. Preferably, the amount of the fluorine-containing carboxylic acid contained is 0.01 vol % or more based on the total amount of the hydrogen fluoride and the fluorine-containing carboxylic acid. Examples of the fluorine-containing carboxylic acid are monofluoroacetic acid, difluoroacetic acid, trifluoroacetic acid, difluoropropionic acid, pentafluoropropionic acid, pentafluorobutyric acid and the like. This dry etching method enables etching of the silicon nitride at a high etching rate and shows a high selectivity ratio of the silicon nitride to silicon oxide and polycrystalline silicon while preventing damage to the silicon oxide.

FIELD OF THE INVENTION

The present disclosure relates to a dry etching method for etching, witha HF-containing gas, silicon nitride (SiN) as an etching target.

BACKGROUND ART

In the case of a semiconductor device having a structure in whichsilicon nitride (hereinafter sometimes abbreviated as “SiN”) is locatedadjacent to silicon oxide (hereinafter sometimes abbreviated as “SiO₂”)and/or polycrystalline silicon (hereinafter sometimes abbreviated as“p-Si”) on a single crystal silicon substrate, the manufacturing of thesemiconductor device includes selective etching of SiN.

Wet etching using hot phosphoric acid and dry etching using a plasmagenerated from a compound gas such as CF₄ are known as methods ofetching SiN.

For example, Patent Document 1 discloses a dry etching method using anetching gas containing a gas of a compound represented by the formula:CH_(x)F_(4-x) (where x is 2 or 3), an oxygen gas etc. for selectiveetching of SiN in the presence of SiO₂, metal silicide or silicon. It isspecifically described in Patent Document 1 that the dry etching methodis adapted for selectively etching a SiN film through an opening of aSiO₂ film while using a p-Si film under the SiN film as an etch stoplayer.

In the wet etching using hot phosphoric acid and in the dry etchingusing a plasma, however, not only SiN but also SiO₂ are etched. Theseetching techniques thus face the problem that it is difficult to ensurethe selectivity ratio of SiN to SiO₂.

Then, Patent Document 2 discloses a method of etching a SiN film formedon a SiO₂ film by feeding a HF gas in a plasma-less heating environment.

As a solution to the problem of a low etching rate of the SiN film onthe SiO₂ film in the etching method of Patent Document 2, PatentDocument 3 discloses the addition of a F₂ gas to the HF gas.

When the SiN film is etched with the HF gas as disclosed in PatentDocument 2, the SiO₂ film is also etched with HF and NH₃ generated as areaction product so that it is not possible to attain a high selectivityratio SiN/SiO₂. When the F₂ gas is added as disclosed in Patent Document3, p-Si is etched with F₂ and the like so that it is not possible toattain a high selectivity ratio SiN/Si.

Patent Document 4 then discloses a method of etching a SiN film with amixed gas of HF and NO at a high selectivity ratio relative to a SiO₂film and/or p-Si film. It is specifically described in this patentdocument that the addition of NO as an etching gas (assist gas) allowsprevention of damage to the SiO₂ film.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Laid-Open Patent Publication No. H8-59215

Patent Document 2: Japanese Laid-Open Patent Publication No. 2008-187105(also published as Japanese Patent No. 4833878)

Patent Document 3: Japanese Laid-Open Patent Publication No. 2010-182730(also published as Japanese Patent No. 5210191)

Patent Document 4: Japanese Laid-Open Patent Publication No. 2014-197603(also published as Japanese Patent No. 6073172)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The etching method of Patent Document 4 provides an improvement in theselectivity ratio of SiN, but cannot sufficiently prevent damage to theSiO₂ film because of the generation of NH₃ as a by-product duringetching of SiN. With the recent progress of fine processing, a surfaceroughness caused by such damage is becoming a non-negligible problem.There has been a demand for improvements to avoid this damage problem.

For prevention of damage to SiO₂, it is conceivable to dilute the mixedetching gas of HF and NO with the addition of an inert gas such as N₂,Ar or He. However, this technique presents a new problem that theoriginally intended etching rate of SiN becomes significantly lowered.In order to complement such a low etching rate, it is conceivable tolengthen the processing time of the etching step and thereby secure theetching amount. The time of exposure of the SiO₂ to the etching gashowever increases with increase in processing time, which results in thedevelopment of damage to the SiO₂ surface. Consequently, the addition ofthe inert gas does not arrive at a solution to the damage problem.

In view of the foregoing, it is an object of the present disclosure toprovide a dry etching method capable of etching SiN at a high etchingrate and, in the case of manufacturing a semiconductor device on asilicon substrate, achieving a high selectivity ratio of SiN to SiO₂ orp-Si and preventing damage to SiO₂.

Means for Solving the Problems

As a result of extensive researches, the present inventors have foundthat: a fluorine-containing carboxylic acid has the capabilities oftrapping NH₃ generated as a by-product during etching of SiN with HF,causing no etching of SiO₂ and p-Si, and not interfering etching of SiNwith HF; and it is possible to solve the above-mentioned problems byetching SiN with a gas containing HF and the fluorine-containingcarboxylic acid. The present disclosure has been accomplished based onthis finding.

Accordingly, the present disclosure provides a dry etching method ofetching silicon nitride, comprising: bringing a mixed gas containinghydrogen fluoride and a fluorine-containing carboxylic acid into contactwith the silicon nitride in a plasma-less process at a temperature lowerthan 100° C.

Effects of the Invention

The present disclosure provides the effect of etching SiN at a highetching rate and, in the case of manufacturing a semiconductor device ona silicon substrate, achieving a high selectivity ratio of SiN to SiO₂and p-Si and preventing damage to SiO₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a reaction device used in Examples andComparative Examples.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present disclosure will be described in detail below.It should be understood that: the following description of features ofthe present disclosure is merely embodiments of the present disclosureand is not intended to limit the present disclosure to theseembodiments; and various changes and modifications can be made to theembodiments within the sprit and scope of the present disclosure.

A dry etching method according to the present disclosure is for etchingsilicon nitride by using a mixed gas containing hydrogen fluoride and afluorine-containing carboxylic acid as a dry etching gas composition andbringing this dry etching gas composition into contact with the siliconnitride in a plasma-less process at a temperature lower than 100° C.

The content amount of the fluorine-containing carboxylic acid in themixed gas is preferably 0.01 vol % or more based on the total amount ofthe hydrogen fluoride and the fluorine-containing carboxylic acid.

The upper limit of the content amount of the fluorine-containingcarboxylic acid is naturally determined depending on the vapor pressuresof the respective compounds and the process pressure. When the processpressure is lower than the vapor pressure of the fluorine-containingcarboxylic acid, the concentration of the HF decreases as the contentamount of the fluorine-containing carboxylic acid becomes large. Thisleads to an insufficiency of the HF, which makes it impossible to ensurethe sufficient etching rate of SiN. It is thus preferable to control themaximum content amount of the fluorine-containing carboxylic acid suchthat the concentration ratio of the HF to the fluorine-containingcarboxylic acid (HF/fluorine-containing carboxylic acid) is 1 or higher.

The content amount of the fluorine-containing carboxylic acid in themixed gas is more preferably 0.01 vol % to 50 vol %, still morepreferably 0.1 vol % to 30 vol %, yet more preferably 3 vol % to 15 vol%, based on the total amount of the hydrogen fluoride and thefluorine-containing carboxylic acid.

Examples of the fluorine-containing carboxylic acid usable in the dryetching method according to the present disclosure are monofluoroaceticacid (CH₂FCOOH), difluoroacetic acid (CHF₂COOH), trifluoroacetic acid(CF₃COOH), difluoropropionic acid (CH₃CF₂COOH), pentafluoropropionicacid (C₂F₅COOH), heptafluorobutyric acid (C₃F₇COOH) and the like. Thesecarboxylic acid gases are preferred because each can be supplied as agas that exhibits an acid dissociation constant pKa lower than or equalto 3.2, which is an acid dissociation constant of the HF, so as to allowpreferential trapping of NH₃, has a certain vapor pressure in thetemperature range of 20 to 100° C. and does not get decomposed in thistemperature range. It is feasible to vaporize the fluorine-containingcarboxylic acid by heating, depressurization, bubbling etc. and supplythe fluorine-containing carboxylic acid in vaporized form.

Although the fluorine-containing carboxylic acid is not necessarily ananhydride, the content of water in the fluorine-containing carboxylicacid is preferably lower than 1 mass %. It is because, when the watercontent of the fluorine-containing carboxylic acid is high, thefluorine-containing carboxylic acid generates H₂O by vaporizationthereof so that there may occur etching of SiO₂ by combination of the HFand H₂O.

The mixed gas may contain, as a dilute gas, an inert gas that does notreact with the HF or fluorine-containing carboxylic acid. It is feasibleto adjust the etching rate of the SiN according to the content amount ofthe inert gas in the mixed gas. Examples of the inert gas are N₂, He,Ne, Ar, Kr and the like. The content amount of the inert gas in themixed gas is generally in the range of 0 vol % to 90 vol %.

The process temperature at which the silicon nitride and the dry etchinggas composition are brought into contact with each other is preferablyhigher than or equal to 20° C. and lower than 100° C., more preferablyhigher than or equal to 40° C. and lower than or equal to 80° C., stillmore preferably higher than or equal to 50° C. and lower than or equalto 75° C.

The process pressure is preferably in the range of 0.1 kPa to 101.3 kPa,more preferably 1 kPa to 50 kPa.

The silicon nitride as the etching target of the present disclosurerefers to a compound represented by SiN_(x) (where x is greater than 0and smaller than or equal to 2) such as Si₃N₄.

It is preferable that, in the case where the dry etching gas compositionaccording to the present disclosure is brought into contact with siliconnitride, silicon oxide and polycrystalline silicon, the SiN-to-SiO₂etching selectivity ratio (SiN/SiO₂) and the SiN-to-p-Si etchingselectivity ratio (SiN/Si) are each 100 or higher. Further, it ispreferable that the etching rate of SiN is at a high level of 100 nm/minor higher.

The dry etching method according to the present disclosure enableshigh-rate, high-selectivity etching of the SiN without causing damage tothe SiO₂ and p-Si. Moreover, the dry etching method according to thepresent disclosure can be implemented in a plasma-less process at a lowtemperature of lower 100° C.

When NH₃ is generated as a by-product during the etching of the SiN,there occurs a side reaction in which the NH₃ reacts with the HF to formNH₄F. This leads to a decrease in the concentration of the HF at the SiNsurface and thus becomes a cause of lowering the etching rate of theSiN. In the dry etching method according to the present disclosure,however, it is expected that the occurrence of the above side reactionis prevented by the addition of the fluorine-containing carboxylic acidwhereby the lowering of the etching rate is suppressed.

In the case where a trace amount of water is contained in the HF or inthe case where absorbed water is present on the SiO₂ surface, theetching of the SiO₂ may proceed by the combined action of the tracewater and HF. In the dry etching method according to the presentdisclosure, it is expected that the trace water is removed by theaddition of the fluorine-containing carboxylic acid whereby the etchingof the SiO₂ is further prevented.

In the case of manufacturing a semiconductor device on a siliconsubstrate, the dry etching method according to the present disclosure isapplicable to the selective etching of SiN from the structure in whichSiN is adjacent to SiO₂ and/or p-Si or in which SiO₂ and/or p-Si and SiNare exposed. Examples of such a structure are those in which a SiO₂and/or p-Si film is covered by a SiN film and in which a SiO₂ film, aSiN film and a p-Si film are laminated to one another. For example, thedry etching method according to the present disclosure can be applied tothe manufacturing of a three-dimensional memory by forming a throughhole in a laminated film of SiO₂ and SiN, supplying the etching gascomposition through the through hole and thereby, while leaving theSiO₂, selectively etching the SiN such that the three-dimensional memoryhas a configuration in which a plurality of SiO₂ layers are arranged inparallel to each other with a clearance held therebetween.

EXAMPLES

The present disclosure will be described in more detail below by way ofthe following examples and comparative examples. It should be understoodthat the present disclosure is not limited to the following examples.

FIG. 1 is a schematic view of a reaction device 1 used in each of theexamples and comparative examples. In the reaction device, a stage 3with a heater function was arranged in a chamber 2. A heater was alsodisposed around the chamber 2 so as to heat a wall of the chamber. A gassupply unit was arranged to supply a dry etching gas composition intothe chamber 2 although not specifically shown in the drawing. A gasintroduction hole 5 was provided on an upper part of the chamber 2 suchthat the dry etching gas composition was introduced into the chamberthrough the gas introduction hole 5 and brought into contact with asample 4 placed on the stage 3. The gas inside the chamber 2 wasdischarged through a gas discharge line 6. Although not specificallyshown in the drawing, a vacuum exhaust pump (as a vacuum exhaust unit)is connected to the gas discharge line so as to depressurize the insideof the chamber 2. Further, a pressure gauge 7 is disposed on the chamber7.

Example 1

As the sample 4, a silicon wafer A with a p-Si film, a silicon wafer Bwith a SiO₂ film and a silicon wafer C with a SiN film were placed onthe stage 3. Herein, each of the SiN film and the p-Si film was formedby a CVD method; and the SiO₂ film was formed by performing thermaloxidation treatment on a surface of the silicon wafer. The temperatureof the stage 3 was set to 70° C. A mixed gas of HF and CF₃COOH (asprepared by mixing 99.9 vol % of HF with 0.1 vol % of CF₃COOH) was fedin a total amount of 1000 scm to the sample. The pressure inside thechamber 2 was set to 10 kPa. The sample was then subjected to etching.

After the etching, the etching rate was respectively determined fromchanges in thicknesses of the p-Si film of the silicon wafer A, the SiO₂film of the silicon wafer B and the SiN film of the silicon wafer Cbefore and after the etching. The SiN-to-p-Si etching rate ratioSiN/p-Si and the SiN-to-SiO₂ etching rate ratio SiN/SiO₂ were alsodetermined.

Furthermore, the surface roughness Ra of the SiO₂ film was evaluated bymeasurement with an atomic force microscope (AFM). The term “surfaceroughness Ra” as used herein refers to an athematic average roughnessaccording to JIS B 0601:1994.

Examples 2 to 5 and Comparative Examples 1 to 3

The etching test and evaluation were carried out in the same manner asin Example 1, except that the kind and concentration of the additive gaswere changed.

The etching conditions and evaluation results of Examples 1 to 5 andComparative Examples 1 to 3 are shown in TABLE 1.

TABLE 1 Kind Conc. Conc. SiN- Surface of [vol %] of [vol %] etchingroughness Process Process additive additive of rate SiN/ Ra [μm] ofpressure temp. gas gas HF [nm/min] p-Si SiN/SiO₂ SiO₂ film Example 1 10kPa 70° C. CF₃COOH 0.1 99.9 839 >1000 156 <1 Example 2 10 kPa 70° C.CF₃COOH 1 99 794 >1000 181 <1 Example 3 10 kPa 70° C. CF₃COOH 5 95729 >1000 281 <1 Example 4 10 kPa 70° C. CF₃COOH 10 90 554 >1000 241 <1Example 5 10 kPa 70° C. C₂F₅COOH 5 95 712 >1000 264 <1 Comparative 10kPa 70° C. none 0 100 845 >1000 82 2.2 Example 1 Comparative 10 kPa 70°C. F₂ 1 99 1004 2 1674 <1 Example 2 Comparative 10 kPa 70° C. NO 10 90699 >1000 233 3.6 Example 3

In Examples 1 to 5, the SiN film was selectively etched as compared tothe p-Si and SiO₂ films. Since there was almost no damage to the surfaceof the SiO₂ film, the surface of the SiO₂ film was smaller than 1 μm inroughness Ra and was very smooth.

On the other hand, in Comparative Example 1 where the etching was doneonly with the HF gas as in Patent Document 2, not only the SiN film butalso the SiO₂ film were etched so that the etching rate ratio SiN/SiO₂was low. In Comparative Example 2 where the etching was done with themixed gas of HF and F₂ as in Patent Document 3, the p-Si film was etchedwith the F₂ gas so that the etching rate ratio SiN/p-Si was low. InComparative Example 3 where the etching was done with mixed gas of HFand NO as in Patent Document 4, there occurred damage to the SiO₂ filmso that the surface of the SiO₂ film after the etching was rough.

DESCRIPTION OF REFERENCE NUMERALS

1: Reaction device

2: Chamber

3: Stage

4: Sample

5: Gas introduction port

6: Gas discharge line

7: Pressure gauge

The invention claimed is:
 1. A dry etching method for selectivelyetching silicon nitride, from a structure in which the silicon nitrideis adjacent to either or both of silicon oxide and polycrystallinesilicon or a structure in which the silicon nitride and either or bothof the silicon oxide and the polycrystalline silicon are exposed, thedry etching method comprising bringing a mixed gas containing hydrogenfluoride and a fluorine-containing carboxylic acid into contact with thesilicon nitride and the either or both of the silicon oxide and thepolycrystalline silicon in a plasma-less process at a temperature lowerthan 100° C., thereby selectively etching the silicon nitride, whereinan etching selectivity ratio of the silicon nitride to the silicon oxideand an etching selectivity ratio of the silicon nitride to thepolycrystalline silicon are each 100 or higher.
 2. The dry etchingmethod according to claim 1, wherein the fluorine-containing carboxylicacid is contained in an amount of 0.01 vol % or more based on the totalamount of the hydrogen fluoride and the fluorine-containing carboxylicacid.
 3. The dry etching method according to claim 1, wherein thefluorine-containing carboxylic acid is at least one kind selected fromthe group consisting of monofluoroacetic acid, difluoroacetic acid,trifluoroacetic acid, difluoropropionic acid, pentafluoropropionic acidand heptafluorobutyric acid.
 4. The dry etching method according toclaim 1, wherein the fluorine-containing carboxylic acid istrifluoroacetic acid or pentafluoropropionic acid, and wherein thefluorine-containing carboxylic acid is contained in an amount of 0.1 vol% to 30 vol % based on the total amount of the hydrogen fluoride and thefluorine-containing carboxylic acid.
 5. The dry etching method accordingto claim 2, wherein the fluorine-containing carboxylic acid is containedin an amount of 0.1 vol % to 30 vol % based on the total amount of thehydrogen fluoride and the fluorine-containing carboxylic acid.
 6. Thedry etching method according to claim 1, wherein the etching of thesilicon nitride is performed at an etching rate of 100 nm/min or higher.7. A dry etching method for selectively etching, from a structure inwhich silicon nitride is adjacent to either or both of silicon oxide andpolycrystalline silicon or a structure in which silicon nitride andeither or both of silicon oxide and polycrystalline silicon are exposed,the silicon nitride, the dry etching method comprising bringing a mixedgas containing hydrogen fluoride and a fluorine-containing carboxylicacid into contact with the silicon nitride and the either or both of thesilicon oxide and the polycrystalline silicon in a plasma-less processat a temperature lower than 100° C., thereby selectively etching thesilicon nitride, wherein the fluorine-containing carboxylic acid is atleast one kind selected from the group consisting of monofluoroaceticacid, difluoroacetic acid, trifluoroacetic acid, difluoropropionic acid,pentafluoropropionic acid and heptafluorobutyric acid, and is containedin an amount of 0.01 vol % or more based on the total amount of thehydrogen fluoride and the fluorine-containing carboxylic acid, andwherein an etching selectivity ratio of the silicon nitride to thesilicon oxide and an etching selectivity ratio of the silicon nitride tothe polycrystalline silicon are each 100 or higher.